In copending Application Ser. No. 313,463, filed Dec. 8, 1972, and now U.S. Pat. No. 3,876,505, which is incorporated herein by reference, there is disclosed a method (hereinafter referred to as the basic method of manufacture) of preparing activated carbon comparable to conventional granular activated carbon, but without the usual steps of pulverizing, mixing, briquetting and carbonizing.
The basic method, to which the improvement of the present invention is directed, requires first of all a proper choice of coal starting material. The method requires a low-grade coal, that is, a coal which is largely nonagglomerating, has high volatile matter, bed moisture greater than 10 percent by weight, an oxygen content greater than 10 percent by weight (other than that contributed by the moisture), and relatively low Btu content. Suitable coals are those ranked as High Volatile C Bituminous Coal, as Subbituminous A, B, and C, and as Lignite A and B Coal in "Classification of Coals by Rank", American Standards Association, ASTM-D388-64T (1964). The bed moisture of the coal employed should be from about 10 to about 25 percent by weight; and the oxygen content from about 10 to about 25 percent by weight, both on a dry, mineral-matter-free basis. The Btu value should be no greater than about 11,000 per pound, on a moist, mineral-matter-free basis. The coal starting material should be a mined, naturally occurring product which has not been treated or processed in such a way as to alter its inherent properties.
Once selected, the coal is then crushed and separated to a size in the range of 2 .times. 40 U.S. Sieve Series. The crushed and separated coal is then heated in air at a temperature of from about 150.degree. C. to about 215.degree. C. for a period of from 30 minutes to 18 hours, whereby from about 1 percent to about 3 percent by weight of oxygen is added to the coal. The thus oxidized coal is then activated at a temperature between 540.degree. C. and 1,100.degree. C. in a controlled oxygen atmosphere.
It has been found that the activated carbon preparation method just described is improved by acid washing of the coal starting materials prior to the step of oxidation by air heating.
Acid treatment of coal for various purposes is well known. For example, Aldrich et al. U.S. Pat. No. 3,816,826 discloses a method for extracting coal from deep mines with the aid of comminuting agents, included among which is glacial acetic acid. Urbanic et al. U.S. Pat. No. 3,778,387 discloses a method for preparing activated carbon with anti-oxidant properties prepared by washing activated carbon with a mineral acid prior to oxidizing. Acids have been used as activation agents. U.S. Pat. Nos. 1,438,113 and 2,437,174 describe the use of boric acid for this purpose, and Netherlands Pat. No. 79,398 describes the use of nitric acid for this purpose. U.S. Pat. No. 3,630,959 describes treatment of bituminous coal with concentrated sulfuric acid, which permits carbonization of the coal at an accelerated rate without caking during heating at 400.degree.-600.degree. C. The product is then steam activated. Russian Pat. No. 233,633 discloses preparation of granular activated carbon by activating a mixture of coal dust and binder with steam, followed by treatment of the product with a hydrochloric acid solution. Acids have been employed to remove the ash (mineral) content of coals. Wortz U.S. Pat. No. 2,036,380 discloses a method of preparing an ash-free activated carbon by maintaining a solution comprising essentially sucrose in concentrated hydrochloric acid at a moderate temperature to form a hard carbon mass which is then ignited to decompose residual organic compounds. Finally, Kitagawa in Kogyo Kagaku Zasshi, 74 (10), 1981-6 and 74 (7), 1383-6 (197l), discloses treatment of chars with dilute hydrochloric acid in order to obtain active carbons with larger specific surface areas when subsequently steam activated.